In fabrication of fluorescent lamps, a blended phosphor layer is applied to the interior surface of a glass envelope using a liquid paint-like suspension of phosphor powder. The phosphor layer may be applied directly to the glass or may alternatively be applied to a previously applied coating of, for example, phosphor or reflective material.
Typically the phosphor blend is a triphosphor blend of a red phosphor, a green phosphor, and a blue phosphor formulated to produce white light output from a fluorescent lamp. A typical blend may include phosphor Types 2345, 2297 and 2461 manufactured by the Chemical & Metallurigical Division of GTE Products Corporation, Towanda, Pa. Type 2345, Y.sub.2 O.sub.3 :Eu, is made by firing a commercially obtained mixture of coprecipitated Yttrium (III) Oxide and Europium (III) Oxide, and then milling and sieving the fired material to produce the finished phosphor powder. This phosphor powder typically comprises tiny crystallites which have been agglomerated into particles having an average size of 3 to 3.5 micrometers as measured by Coulter Counter. Type 2297 is a Cerium Terbium Magnesium Aluminate: Cerium: Terbium phosphor, and 2461 is a Barium Magnesium Aluminate: Europium phosphor. Typical formulae for the 2461 and the 2297 are BaMg.sub.2A Al.sub.16 O.sub.27 :Eu (blue) and (Ce,Tb)MgAl.sub.11 O.sub.19 :Ce:Tb (green), respectively.
Although the composition of the phosphor coating suspension may vary from lamp manufacturer to lamp manufacturer, the suspension usually includes, in addition to the phosphor, a film forming binder, solvent(s) for the binder, and, if necessary, surfactants and defoamers. The coating suspension may further include submicron particle size alumina, e.g., Aluminum Oxide C, manufactured by Degussa, Inc. In GTE triphosphor coating suspensions, an Aluminum Oxide C content of 2 to 10% by weight of the phosphor is typical. The submicron particle size alumina assists in the formation of a uniform adherent phosphor layer on the lamp surface.
A problem with such aqueous blends of lamp phosphors is that the red-emitting phosphor Y.sub.2 O.sub.3 :Eu may undergo degradation in the water base phosphor coating suspension. The degradation is caused by dissolution and subsequent precipitation of the europium activated yttrium oxide phosphor in the water base suspension system. Dissolution of europium activated yttrium oxide causes further concerns when it is used as a component of a multicomponent blend, such as a triphosphor blend. The dissolution is caused by a drop in suspension pH resulting from the addition of Aluminum Oxide C which has acidic impurities. As the pH subsequently rises due to the inherent basic nature of the yttrium oxide phosphor, the dissolved yttrium and europium ions precipitate out in the form of a scum layer onto the surface of the phosphor particles. The scum layer precipitate contains mainly yttrium hydroxide and organometallic compounds, the latter evidenced by the interaction of the yttrium, europium and aluminum ions and the polymeric binder. The scum layer does not bake out within the normal temperature range of the ovens. This leads to a discolored lamp which is unacceptable.
Before a production run is made, small quantities of test lamps are made with the coating suspension. Once a multicomponent lamp suspension containing the yttrium oxide phosphor has become degraded, an unacceptable body color results after tests lamps coated with the degraded aqueous suspension are baked. So as to minimize environmental concerns for disposal of degraded phosphor suspensions, it is desirable to economically utilize the degraded phosphor suspensions. Heretofore, a method has not been developed for removing and reclaiming valuable components of the degraded aqueous suspension while producing a resulting phosphor blend that may be used for the manufacture of lamps free from discoloration.